Solar Luminosity Variations over Timescales of Days to the past Few Solar Cycles

Abstract

Dips in the total irradiance of up to 0.2% and lasting typically 10 days are now well known to be caused by the transit of dark sunspots across the photospheric disc. The large bright magnetic faculae usually associated with spots cause irradiance increases of comparable magnitude although the form of their signal is more subtle. Radiometry from five satellites beginning in late 1978 indicates a minimum in irradiance at the epoch of lowest magnetic activity between solar cycles 21 and 22. Analysis of these radiometric measurements indicates that this irradiance decline between about 1981 and 1986 was caused mainly by decay in the excess radiation of bright faculae in the magnetic network outside of active regions. Empirical models of irradiance modulation extending back to 1874 indicate that the Sun is typically about 0.05% brighter at activity maximum than at minimum.